This invention relates generally to electrical telecommunication and more particularly to synchronizing transceivers of different users and even more particularly to methods and apparatus for synchronization based on orthogonal sequences having optimized correlation properties.
Modem communication systems, such as cellular and satellite radio systems, employ various modes of operation (analog, digital, and hybrids) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
Digital cellular communication systems have expanded functionality for optimizing system capacity and supporting hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term xe2x80x9cmacrocellxe2x80x9d generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms xe2x80x9cmicrocellxe2x80x9d and xe2x80x9cpicocellxe2x80x9d generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments.
FIG. 1 illustrates an exemplary hierarchical, or multi-layered, cellular system. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella cell may contain an underlying microcell structure. The umbrella cell 10 includes microcell 20 represented by the area enclosed within the dotted line and microcell 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 20 and 30 may be an area of dense traffic concentration, and thus might represent a hot spot.
FIG. 2 is a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station (BS) 110 and mobile station (MS) 120. The BS includes a control and processing unit 130 which is connected to a mobile switching center (MSC) 140 which in turn is connected to the public switched telephone network (PSTN) (not shown). General aspects of such cellular radiotelephone systems are known in the art. The BS1110 handles a plurality of voice channels through a voice channel transceiver 150, which is controlled by the control and processing unit 130. Also, each BS includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the BS or cell to MSs locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with control and traffic channels that share the same radio carrier.
The MS 120 receives the information broadcast on a control channel at its voice and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the MS to lock on to, and determines on which cell the MS should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described for example in U.S. Pat. No. 5,353,332 to Raith et al., entitled xe2x80x9cMethod and Apparatus for Communication Control in a Radiotelephone Systemxe2x80x9d.
In North America, a digital cellular radiotelephone system using TDMA is called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence CDMA (DS-CDMA) is specified by the TIA/EIA/IS-95 standard, and a frequency hopping CDMA communication system is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan""s Association of Radio Industries and Businesses (ARIB). Besides transmitting voice information, the next generation systems can be expected to carry packet data and to inter-operate with packet data networks that are also usually designed and based on industry-wide data standards such as the open system interface (OSI) model or the transmission control protocol/Internet protocol (TCP/IP) stack. These standards have been developed, whether formally or de facto, for many years, and the applications that use these protocols are readily available. The main objective of standards-based networks is to achieve interconnectivity with other networks. The Internet is today""s most obvious example of such a standards-based packet data network in pursuit of this goal.
In most of these digital communication systems, communication channels are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz), 900 MHz, 1800 MHz, and 1900 MHz. In TDMA systems and even to varying extents in CDMA systems, each radio channel is divided into a series of time slots, each of which contains a block of information from a user. The time slots are grouped into successive frames that each have a predetermined duration, and successive frames may be grouped into a succession of what are usually called superframes. The kind of access technique (e.g., TDMA or CDMA) used by a communication system affects how user information is represented in the slots and frames, but current access techniques all use a slot/frame structure.
Time slots assigned to the same user, which may not be consecutive time slots on the radio carrier, may be considered a logical channel assigned to the user. During each time slot, a predetermined number of digital bits are transmitted according to the particular access technique (e.g., CDMA) used by the system. In addition to logical channels for voice or data traffic, cellular radio communication systems also provide logical channels for control messages, such as paging/access channels for call-setup messages exchanged by BSs and MSs and synchronization channels for broadcast messages used by MSs and other remote terminals for synchronizing their transceivers to the frame/slot/bit structures of the BSs. In general, the transmission bit rates of these different channels need not coincide and the lengths of the slots in the different channels need not be uniform. Moreover, third generation cellular communication systems being considered in Europe and Japan are asynchronous, meaning that the structure of one BS is not temporally related to the structure of another BS and that an MS does not know any of the structures in advance.
In such digital communication systems, a receiving terminal must find the timing reference of a transmitting terminal before any information transfer can take place. For a communication system using DS-CDMA, finding the timing reference corresponds to finding the boundaries of downlink (e.g., BS-to-MS) chips, symbols, and frames. These are sometimes called downlink chip-, symbol-, and frame-synchronizations, respectively. In this context, a frame is simply a block of data that can be independently detected and decoded. Frame lengths in today""s systems typically fall in the range of ten milliseconds (ms) to twenty ms. This search of BS timing may be called a xe2x80x9ccell searchxe2x80x9d, and it includes identification of BS-specific downlink scrambling codes that are features of current DS-CDMA communication systems.
An MS or other remote terminal typically receives a signal that is a superposition (sum) of attenuated, faded, and disturbed versions of the signal transmitted by a BS. The slot and frame boundaries in the received signal are unknown to the MS to begin with, as are any BS-specific scrambling codes. The goal of the MS is thus to detect and identify one or more BSs in the noise-like (for DS-CDMA) received signal and to identify the scrambling code used.
In order to help synchronize the remote terminal to the BS and identify the BS-specific scrambling code, some communication systems provide that each BS signal includes an unscrambled part, which may be called a synchronization channel SCH, which the MS can lock onto and perform a cell search. Applicant""s invention improves such synchronization channels in terms of both performance and MS complexity.
In one aspect of Applicant""s invention, a method of determining a scrambling code group for a received signal in a digital communication system is provided. Signals in the communication system are scrambled by respective scrambling codes; the scrambling codes are assigned to respective scrambling code groups; and identities of the scrambling code groups are encoded in the signals by respective cyclically distinct sequences of signed code words that are S-Hadamard sequences. The method includes the steps of: correlating the received signal to each of a plurality of the code words; coherently combining the correlations in accordance with cyclic shifts of each of a plurality of sequences of signs; and determining a maximal coherently combined correlation to identify the scrambling code group for the received signal.
In another aspect of Applicant""s invention, a method of determining a scrambling code group for a received signal in a digital communication system, in which signals are scrambled by respective scrambling codes, the scrambling codes are assigned to respective scrambling code groups, identities of the scrambling code groups are encoded in the signals by respective cyclically distinct sequences of code words, is provided. The method includes the steps of: correlating the received signal to cyclic shifts of each of a plurality of sequences of code words that are S-Hadamard sequences; combining the correlations for each of the plurality of sequences of code words; and determining a maximal combined correlation to identify the scrambling code group for the received signal.
In another aspect of Applicant""s invention, a digital radio transmission system having at least one transmitter and at least one receiver includes a device in the transmitter for generating a synchronization signal that includes signed versions of S-Hadamard sequences. The S-Hadamard sequences are obtained by position-wise scrambling a Walsh-Hadamard sequence with a special sequence having complex elements of constant magnitude. There is also included a device in the receiver forxe2x80x94estimating a time location and sequence identity of a received version of the synchronization signal.
In another aspect of Applicant""s invention, a method of determining a time location of a received signal and identifying a Walsh-Hadamard sequence encoded as a S-Hadamard sequence included in the received signal is provided. The S-Hadamard sequence is a product of the Walsh-Hadamard sequence and a special sequence having complex elements of constant magnitude, and the Walsh-Hadamard sequence is a member of a first set of Walsh-Hadamard sequences. The method includes the steps of: forming a product of the received signal and the special sequence; and correlating the product with each of a plurality of Walsh-Hadamard sequences for identifying the Walsh-Hadamard sequence encoded in the received signal.